Hey moldie, just make sure you dont buy a Geforce 4 MX model, if your going to buy Geforce 4 make it a Geforce 4 Ti (titanium) 4200 or higher, or Geforce FX 5600 or higher, if you buy anything lower than that your wasting your hard earned money. "quote me on this, dont buy MX.. no matter what anyone says" ok take care peace.
In the BIOS (accessed either by Delete or F2) under the Advanced BIOS Features or Advanced Chipset Features there should be an option resembling "Video Memory" or "AGP Memory". Set this as high as you feel fit but remember that it reserves that space for itself and does not share it with any other program. This will make your memory seem smaller then it is in Windows.